"Not the Tour de France, but there's a lot of other things I could do.

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"I made my bed," he said. "But if there was ever a window, would I like to run the Chicago Marathon when I'm 50? I would love to do that."

In the opening segment of the interview shown Thursday, Armstrong confessed that his record seven Tour de France titles were fuelled by drugs, confirming much of the US Anti-Doping Agency's findings about his use of blood-boosting EPO, blood doping, cortisone, testosterone and human growth hormone.

Despite that, Armstrong said he believes he should have a chance to return to competition. "I don't expect it to happen," he acknowledged.

"Frankly, this may not be the most popular answer, but I think I deserve it," he said, telling Winfrey that former team-mates who implicated themselves in testifying against him received lesser punishments.

"I deserve to be punished," Armstrong said. "I'm not sure that I deserve a death penalty."

The first installment of the interview was a ratings winner for Winfrey, with its estimated 3.2 million viewers in the United States making it the second-most-watched show ever on her fledgling OWN network.

However, it left many still sceptical of Armstrong's motives and methods, doubtful that he felt real remorse.

Genuine emotion seeped through on the second installment. Armstrong struggled to keep his composure as he described telling his 13-year-old son Luke: "Don't defend me anymore" when his transgressions caught up with him last year.

"When this all really started, I saw my son defending me and saying, 'That's not true. What you're saying about my dad is not true.'

"That's when I knew I had to tell him," Armstrong said. "And he'd never asked me. He'd never said, 'Dad, is this true?' He trusted me."

Armstrong recalled the days in October, after USADA released the report documenting its case against him, that led to his stepping down as chairman of the Livestrong cancer charity he founded and then leaving the board entirely.

"I wouldn't at all say forced out," Armstrong said. "I was aware of the pressure.

"It was the best thing for the organisation but it hurt like hell... That was the lowest."

He discussed the financial fallout, in particular the stampede of sponsors away from him with sportswear giant Nike in the lead.

"And you could look at the day or those two days or the day and a half where people left. You asked me the cost," he said. "That was a $75 million day."

Armstrong, who told Winfrey he's in therapy, said he doesn't know how his story will turn out -- perhaps one of the most painful admissions for a man so determined to stay in control.

"I do not know the outcome here," he said. "And I'm getting comfortable with that."